In November nineteen thirty-two preparations were made to divert the river through the tunnels. A small blast opened a breech and a temporary dike which had held the river in check at the entrance to one of the tubes, opening the way for the free passage of the water into the fifty foot concrete-lined bore. A temporary dam of earth and rock was quickly thrown across the stream deflecting the entire flow. Within twenty-four hours, the Colorado River under control for the first time in its history was flowing around and past the dam site through the huge diversion tubes.

An all time record was set in placing the more than one million cubic yards of material required for the construction of the two coffer dams, in themselves barriers of no mean proportions. The earth fills were compacted to an elevation well above the high water mark to prevent the flow of the river from entering the scene of operations during the construction of the dam and power plant. After the completion of the coffer dams, the site was unwatered and the fleet of power shovels and trucks put to work on the excavation of the riverbed material to provide foundations for the structures. This excavation was carried to a point a hundred and thirty-five feet below the old river level, necessitating the handling of more than two million yards of rock, earth, and sand. Here again, the stamina of both men and machinery was put to a severe test in transporting the waste from the very lowest depths of the gorge.

As the cleanup of the dam site progressed, the ancient bed of the Colorado River was laid bare. Here, geologists were able to read the story of what had happened ages ago when the chasm now called Black Canyon had been carved out of the primal rock by the rush of water from a great inland sea and the Colorado River was settling into its present bed. To insure the greatest possible stability for the foundation of the dam, meticulous care was exercised in preparing the rock surfaces for the reception of the first concrete.